The invention relates to an electronic fuel injection system for providing a combustible mixture to mixture-compressing internal combustion engines. More especially, the invention relates to an electronic fuel injection system which processes information related to the air flow rate of the engine and the prevailing engine speed (rpm) into fuel injection control pulses, the duration of which determines the amount of fuel which is injected, for example by electromagnetic injection valves located in the vicinity of the engine. The basic control pulse is then usually corrected by various correcting circuits which take account of a multitude of prevailing engine conditions and may include a so-called .lambda.-control, which also affects the final, corrected duration of the fuel injection control pulses. A .lambda.-control process usually includes an oxygen or so-called .lambda.-sensor located in the exhaust channel and associated with electronic integrating circuitry. The signal from the .lambda.-sensor permits conclusions to be made regarding the original composition of the fuel-air mixture so that a .lambda.-sensor signal may be used in a closed-loop, feedback, type of control which makes it possible to adapt the amount of fuel fed to the engine precisely to prevailing conditions. Such a sensitive and precise closed-loop control is very desirable because it reduces fuel consumption and reduces the toxicity of the exhaust gas.
It is a feature of known fuel injection systems to shut off the fuel supply to the engine completely under certain conditions of operation, for example during overrunning, i.e. when the engine delivers negative torque, for example in downhill operation. That type of operation is identified by two conditions, namely a closed throttle valve and a relatively high engine speed. The fuel is shut off by completely suppressing the previously mentioned fuel injection control pulses. In this condition, i.e., when all fuel is shut off, the oxygen sensor located in the exhaust system will indicate a lean mixture and will cause the integrating circuit within the .lambda.-control loop to run up against its enriching limit.
When the engine leaves this state, for example by running slower than the limiting rpm or because the throttle valve is reopened to supply power when the downhill operation is complete, the fuel-air mixture being fed to the engine will be substantially too rich for a prolonged period of time because the duration of the control pulses can adapt to prevailing conditions only at the relatively slow response rate of the integrator.